text: Methionine sulfoxide is the organic compound with the formula CHS(O)CHCHCH(NH)COH.  It is an amino acid that occurs naturally although it is formed post-translationally.
  Oxidation of the sulfur of methionine results in methionine sulfoxide or methionine sulfone. The sulfur-containing amino acids methionine and cysteine are more easily oxidized than the other amino acids. Unlike oxidation of other amino acids, the oxidation of methionine can be reversed by enzymatic action, specifically by enzymes in the methionine sulfoxide reductase family of enzymes. The three known methionine sulfoxide reductases are MsrA, MsrB, and fRmsr. Oxidation of methionine results in a mixture of the two diastereomers methionine-S-sulfoxide and methionine-R-sulfoxide, which are reduced by MsrA and MsrB, respectively. MsrA can reduce both free and protein-based methionine-S-sulfoxide, whereas MsrB is specific for protein-based methionine-R-sulfoxide. fRmsr, however, catalyzes the reduction of free methionine-R-sulfoxide. Thioredoxin serves to recycle by reduction some of the methionine sulfoxide reductase family of enzymes, whereas others can be reduced by metallothionein.